We have a anemometer with RS422/485 communication, and we would like to connect to the PC, but we have only RS232 so I would like to purchase a converter, but some converter has an external power supply, some not. The maximum distance will be less than 100m. I which case do I need an RS422/485-RS232 converter with external power supply?
It really depends on which end of the run the converter is on. If you place it near the PC, so that the 100m run is RS-422, then a converter powered by the PC's serial port (no external supply) will probably work fine. If you want to place the converter near the sensor, so that the 100m run is RS-232, then it would probably be wise to use one that has an external supply.